Special Issue "Environmental Barrier Coatings"

A special issue of Coatings (ISSN 2079-6412).

Deadline for manuscript submissions: closed (31 December 2019).

Special Issue Editor

Dr. Kang N. Lee
E-Mail Website
Guest Editor
NASA Glenn Research Center
Interests: environmental barrier coatings, thermal barrier coatings, high temperature oxidation, high temperature materials chemistry, solid-state batteries

Special Issue Information

Dear Colleagues,

The global increase in air travel will require commercial vehicles to be more efficient than ever before. Advanced engine hot section materials are a key technology required to keep fuel consumption and emission to a minimum in next-generation gas turbines. Ceramic matrix composites (CMCs) are the most promising material to revolutionize gas turbine hot section materials technology because of their excellent high‐temperature properties. Rapid surface recession due to volatilization by water vapor is the Achilles heel of CMCs. Environmental barrier coatings (EBCs) is an enabling technology for CMCs, since it protects CMCs from water vapor. The first CMC component entered into service in 2016 in a commercial engine, and more CMC components are scheduled to follow within the next few years. One of the most difficult challenges to CMC components is EBC durability, because failure of EBC leads to a rapid reduction in CMC component life. Key contributors to EBC failure include recession, oxidation, degradation by calcium‐aluminum‐magnesium silicates (CMAS) deposits, thermal and thermo‐mechanical strains, particle erosion, and foreign object damage (FOD). Novel EBC chemistries, creative EBC designs, and robust processes are required to meet EBC durability challenges. Engine-relevant testing, characterization, and lifing methods need to be developed to improve EBC reliability. The aim of this Special Issue is to present recent advances in EBC technology to address these issues.

In particular, topics of interest include but are not limited to the following:

  • Novel EBC chemistries and designs;
  • Processing including plasma spray, suspension plasma spray, solution precursor plasma spray, slurry process, PS-PVD, EB-PVD, and CVD;
  • Testing, characterization, and modeling;
  • Lifing.

Dr. Kang N. Lee
Guest Editor

Manuscript Submission Information

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Keywords

  • Environmental barrier coating (EBC)
  • Ceramic matrix composite (CMC)
  • Durability
  • Oxidation
  • Lifing

Published Papers (9 papers)

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Research

Open AccessFeature PaperArticle
Relative Ti2AlC Scale Volatility under 1300 °C Combustion Conditions
Coatings 2020, 10(2), 142; https://doi.org/10.3390/coatings10020142 - 05 Feb 2020
Abstract
Turbine environments may degrade high temperature ceramics because of volatile hydroxide reaction products formed in water vapor. Accordingly, the volatility of transient TiO2 and steady-state Al2O3 scales formed on the oxidation-resistant Ti2AlC MAX phase ceramic was examined [...] Read more.
Turbine environments may degrade high temperature ceramics because of volatile hydroxide reaction products formed in water vapor. Accordingly, the volatility of transient TiO2 and steady-state Al2O3 scales formed on the oxidation-resistant Ti2AlC MAX phase ceramic was examined in 1300 °C high velocity (Mach 0.3, 100 m/s) and high pressure (6 atm, 25 m/s) burner rig tests (BRT). Unlike metals, the ceramic was stable at 1300 °C. Unlike SiC and Si3N4, neither burner test produced a weight loss, unless heavily pre-oxidized. Lower mass gains were produced in the BRT compared to furnace tests. The commonly observed initial, fast TiO2 transient scale was preferentially removed in hot burner gas (~10% water vapor). A lesser degree of gradual Al2O3 volatilization occurred, indicated by grain boundary porosity and crystallographic etching. Modified cubic-linear (growth-volatility) kinetics are suggested. Gas velocity and water vapor pressure play specific roles for each scale. Furthermore, a 7YSZ TBC on Ti2AlC survived for 500 h in the Mach 0.3 burner test at 1300 °C with no indication of volatility or spalling. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
Open AccessArticle
Characterization of Thermochemical and Thermomechanical Properties of Eyjafjallajökull Volcanic Ash Glass
Coatings 2020, 10(2), 100; https://doi.org/10.3390/coatings10020100 - 23 Jan 2020
Abstract
The properties of a volcanic ash glass obtained from the Eyjafjallajökull eruption of 2010 were studied. Crystallization experiments were carried out on bulk and powdered glass samples at temperatures between 900 and 1300 °C. Iron oxides, Fe3O4 and Fe2 [...] Read more.
The properties of a volcanic ash glass obtained from the Eyjafjallajökull eruption of 2010 were studied. Crystallization experiments were carried out on bulk and powdered glass samples at temperatures between 900 and 1300 °C. Iron oxides, Fe3O4 and Fe2O3, and a silicate plagioclase, (Na,Ca)(Si,Al)4O8, were observed. Bulk samples remained mostly amorphous after up to 40 h at temperature. Powdered glass samples showed increased crystallinity after heat treatment compared to bulk samples. The average coefficient of thermal expansion of the glass was 7.00 × 10−6 K−1 over 25–720 °C. The Vickers hardness of the glass was 6–7 GPa and the indentation fracture toughness, 1–2 MPa √m Values for density, elastic modulus, and Poisson’s ratio were 2.52 g/cm3, 75 GPa, and 0.24, respectively. The viscosity of the glass was determined experimentally and compared to three common models from the literature. The implications for the deposition of volcanic ash on hot section components of aircraft turbine engines are discussed. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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Open AccessFeature PaperArticle
Delayed Formation of Thermally Grown Oxide in Environmental Barrier Coatings for Non-Oxide Ceramic Matrix Composites
Coatings 2020, 10(1), 6; https://doi.org/10.3390/coatings10010006 - 19 Dec 2019
Abstract
The oxidation and corrosion behavior at elevated temperatures of a SiCF/SiC(N) composite with two plasma-sprayed environmental barrier coating (EBC) systems were studied. After both processes, the formation of a silica-based thermally grown oxide (TGO) layer was observed. The formation of this [...] Read more.
The oxidation and corrosion behavior at elevated temperatures of a SiCF/SiC(N) composite with two plasma-sprayed environmental barrier coating (EBC) systems were studied. After both processes, the formation of a silica-based thermally grown oxide (TGO) layer was observed. The formation of this TGO caused two principal failure mechanisms of the EBC. Firstly, spallation of the EBC induced by stresses from volume expansion and phase transformation to crystalline SiO2 was observed. Water vapor corrosion of the TGO with gap formation in the top region of the TGO was found to be a second failure mechanism. After a burner rig test of the Al2O3-YAG EBC system, this corrosion process was observed at the TGO surface and in the volume of the Al2O3 bond coat. In the case of the second system, Si-Yb2Si2O7/SiC-Yb2SiO5, the formation of the TGO could be delayed by introducing an additional intermediate layer based on Yb2Si2O7 filled with SiC particles. The SiC particles in the intermediate layer were oxidized and served as getter to reduce the permeation of oxidants (O2, H2O) into the material. In this way, the formation of the TGO and the failure mechanisms caused by their formation and growth could be delayed. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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Open AccessFeature PaperArticle
Environmental Barrier Coatings Made by Different Thermal Spray Technologies
Coatings 2019, 9(12), 784; https://doi.org/10.3390/coatings9120784 - 22 Nov 2019
Cited by 1
Abstract
Environmental barrier coatings (EBCs) are essential to protect ceramic matrix composites against water vapor recession in typical gas turbine environments. Both oxide and non-oxide-based ceramic matrix composites (CMCs) need such coatings as they show only a limited stability. As the thermal expansion coefficients [...] Read more.
Environmental barrier coatings (EBCs) are essential to protect ceramic matrix composites against water vapor recession in typical gas turbine environments. Both oxide and non-oxide-based ceramic matrix composites (CMCs) need such coatings as they show only a limited stability. As the thermal expansion coefficients are quite different between the two CMCs, the suitable EBC materials for both applications are different. In the paper examples of EBCs for both types of CMCs are presented. In case of EBCs for oxide-based CMCs, the limited strength of the CMC leads to damage of the surface if standard grit-blasting techniques are used. Only in the case of oxide-based CMCs different processes as laser ablation have been used to optimize the surface topography. Another result for many EBCs for oxide-based CMC is the possibility to deposit them by standard atmospheric plasma spraying (APS) as crystalline coatings. Hence, in case of these coatings only the APS process will be described. For the EBCs for non-oxide CMCs the state-of-the-art materials are rare earth or yttrium silicates. Here the major challenge is to obtain dense and crystalline coatings. While for the Y2SiO5 a promising microstructure could be obtained by a heat-treatment of an APS coating, this was not the case for Yb2Si2O7. Here also other thermal spray processes as high velocity oxygen fuel (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) are used and the results described mainly with respect to crystallinity and porosity. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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Open AccessArticle
Crack Initiation Criteria in EBC under Thermal Stress
Coatings 2019, 9(11), 697; https://doi.org/10.3390/coatings9110697 - 24 Oct 2019
Abstract
For design of multi-layered environmental barrier coatings (EBCs), it is essential to assure mechanical reliability against interface crack initiation and propagation induced by thermal stress owing to a misfit of the coefficients of thermal expansion between the coating layers and SiC/SiC substrate. We [...] Read more.
For design of multi-layered environmental barrier coatings (EBCs), it is essential to assure mechanical reliability against interface crack initiation and propagation induced by thermal stress owing to a misfit of the coefficients of thermal expansion between the coating layers and SiC/SiC substrate. We conducted finite element method (FEM) analyses to evaluate energy release rate (ERR) for interface cracks and performed experiment to obtain interface fracture toughness to assess mechanical reliability of an EBC with a function of thermal barrier (T/EBC; SiC/SiAlON/mullite/Yb-silicate gradient composition layer/Yb2SiO5 with porous segment structure) on an SiC/SiC substrate under thermal stress due to cooling in fabrication process. Our FEM analysis revealed that a thinner SiAlON layer and a thicker mullite layer are most suitable to reduce ERRs for crack initiation at the SiC/SiAlON, SiAlON/mullite and mullite/Yb2Si2O7 interfaces. Interface fracture tests of the T/EBC with layer thicknesses within the proposed range exhibited fracture at the SiC/SiAlON and SiAlON/mullite interfaces. We also estimated the approximate fracture toughness for the SiC/SiAlON and SiAlON/mullite interfaces and lower limit of fracture toughness for the mullite/Yb2Si2O7 interface. Comparison between ERR and fracture toughness indicates that the fabricated T/EBC possesses sufficient mechanical reliability against interface crack initiation and propagation. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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Open AccessArticle
Structural Stabilization of Mullite Films Exposed to Oxygen Potential Gradients at High Temperatures
Coatings 2019, 9(10), 630; https://doi.org/10.3390/coatings9100630 - 30 Sep 2019
Cited by 1
Abstract
The oxygen shielding properties of polycrystalline Al4+2xSi2−2xO10−x (mullite) films applied as environmental barrier coatings (EBCs) on SiC fiber-reinforced SiC matrix composites (SiC/SiC) are determined by the grain boundary (GB) diffusion of oxide ions in the [...] Read more.
The oxygen shielding properties of polycrystalline Al4+2xSi2−2xO10−x (mullite) films applied as environmental barrier coatings (EBCs) on SiC fiber-reinforced SiC matrix composites (SiC/SiC) are determined by the grain boundary (GB) diffusion of oxide ions in the films, from the higher oxygen partial pressure (PO₂) surface to the lower PO₂ surface, with simultaneous GB diffusion of Al ions in the opposite direction. Herein, strategies to improve the oxygen shielding and phase stability of these films when applied to SiC/SiC substrates through bond coats are proposed, based on oxygen permeation data for mullite at high temperatures. The validity of these strategies is verified using experimental trials at 1673 K with bilayer specimens consisting of mullite films and bond coat substrates, serving as model EBCs. The data show that employing a bond coat made of β’-SiAlON rather than Si provides a source of Al for the overlying mullite film that greatly improves the phase stability of the film in the vicinity of the junction interface. Because the minimum equilibrium PO₂ values required to form SiO2 due to oxidation of the β’-SiAlON on a thermodynamic basis are significantly larger than those for oxidation of Si, the inward GB diffusion of oxide ions is effectively retarded, resulting in excellent oxygen shielding characteristics. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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Open AccessArticle
YAlO3—A Novel Environmental Barrier Coating for Al2O3/Al2O3–Ceramic Matrix Composites
Coatings 2019, 9(10), 609; https://doi.org/10.3390/coatings9100609 - 25 Sep 2019
Cited by 1
Abstract
Ceramic matrix composites (CMCs) are promising materials for high-temperature applications. Environmental barrier coatings (EBCs) are needed to protect the components against water vapor attack. A new potential EBC material, YAlO3, was studied in this paper. Different plasma-spraying techniques were used for [...] Read more.
Ceramic matrix composites (CMCs) are promising materials for high-temperature applications. Environmental barrier coatings (EBCs) are needed to protect the components against water vapor attack. A new potential EBC material, YAlO3, was studied in this paper. Different plasma-spraying techniques were used for the production of coatings on an alumina-based CMC, such as atmospheric plasma spraying (APS) and very low pressure plasma spraying (VLPPS). No bond coats or surface treatments were applied. The performance was tested by pull–adhesion tests, burner rig tests, and calcium-magnesium-aluminum-silicate (CMAS) corrosion tests. The samples were subsequently analyzed by means of X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Special attention was paid to the interaction at the interface between coating and substrate. The results show that fully crystalline and good adherent YAlO3 coatings can be produced without further substrate preparation such as surface pretreatment or bond coat application. The formation of a thin reaction layer between coating and substrate seems to promote adhesion. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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Open AccessArticle
Crack Healing in Mullite-Based EBC during Thermal Shock Cycle
Coatings 2019, 9(9), 585; https://doi.org/10.3390/coatings9090585 - 17 Sep 2019
Abstract
Crack healing phenomena were observed in mullite and mullite + Yb2SiO5 environmental barrier coating (EBC) materials during thermal shock cycles. Air plasma spray coating was used to deposit the EBC materials onto a Si bondcoat on a SiCf/SiC [...] Read more.
Crack healing phenomena were observed in mullite and mullite + Yb2SiO5 environmental barrier coating (EBC) materials during thermal shock cycles. Air plasma spray coating was used to deposit the EBC materials onto a Si bondcoat on a SiCf/SiC composite substrate. This study reveals that unidirectional vertical cracks (mud cracks) formed after several thermal shock cycles; however, the cracks were stable for 5000 thermal shock cycles at a maximum temperature of 1350 °C. Moreover, the crack densities decreased with an increasing number of thermal shock cycles. After 3000 thermal shock cycles, cracks were healed via melting of a phase containing SiO2 phase, which partially filled the gaps of the cracks and resulted in the precipitation of crystalline Al2O3 in the mullite. Post-indentation tests after thermal shock cycling indicated that the mullite-based EBC maintained its initial mechanical behavior compared to Y2SiO5. The indentation load–displacement tests revealed that, among the materials investigated in the present study, the mullite + Yb2SiO5 EBC demonstrated the best durability during repetitive thermal shocks. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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Open AccessArticle
Flow Kinetics of Molten Silicates through Thermal Barrier Coating: A Numerical Study
Coatings 2019, 9(5), 332; https://doi.org/10.3390/coatings9050332 - 23 May 2019
Abstract
Infiltration of molten calcium–magnesium–alumina–silicates (CMAS) through thermal barrier coatings (TBCs) causes structural degradation of TBC layers. The infiltration kinetics can be altered by careful tailoring of the electron beam physical vapor deposition (EB-PVD) microstructure such as feather arm lengths and inter-columnar gaps, etc. [...] Read more.
Infiltration of molten calcium–magnesium–alumina–silicates (CMAS) through thermal barrier coatings (TBCs) causes structural degradation of TBC layers. The infiltration kinetics can be altered by careful tailoring of the electron beam physical vapor deposition (EB-PVD) microstructure such as feather arm lengths and inter-columnar gaps, etc. Morphology of the feathery columns and their inherent porosities directly influences the infiltration kinetics of molten CMAS. To understand the influence of columnar morphology on the kinetics of the CAMS flow, a finite element based parametric model was developed for describing a variety of EB-PVD top coat microstructures. A detailed numerical study was performed considering fluid-solid interactions (FSI) between the CMAS and TBC top coat (TC). The CMAS flow characteristics through these microstructures were assessed quantitatively and qualitatively. Finally, correlations between the morphological parameters and CMAS flow kinetics were established. It was shown that the rate of longitudinal and lateral infiltration could be minimized by reducing the gap between columns and increasing the length of the feather arms. The results also show that the microstructures with long feather arms having a lower lateral inclination decrease the CMAS infiltration rate, therefore, reduce the CMAS infiltration depth. The analyses allow the identification of key morphological features that are important for mitigating the CMAS infiltration. Full article
(This article belongs to the Special Issue Environmental Barrier Coatings)
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